The invention relates to asynchronous motors employing a squirrel-cage rotor.
The starting torque of asynchronous machines is normally determined by the loss arising in the rotor at rest. With machines having a simple short circuit rotor the resistance of the rotor is limited by the permissible operational loss. Taking into consideration that the values of resistance of the rotor are identical both in the course of operation and when starting, the loss arising in the rest condition (and accordingly, the starting torque) are resulting values. This resulting value is called the natural starting torque. The proportion of the natural starting torque to the nominal torque will generally be reduced, to increase the output, so that above an output of 50-200 kW it is not enough to run a working machine, which can be started otherwise easily.
Special measures are to be taken in order to increase the resistance required at starting and thus the loss arising in course of starting, while maintaining the operational resistance at a constant level. For this purpose, in order to provide asynchronous motors with a high output, rotors with double cages or with deep bars are used. In both cases the phenomenon of current displacement is exploited. In the former case the current is unevenly distributed between two cages, while in the latter case uneven distribution may be observed within the bar. Both can be characterized by the fact that the increase of losses at start takes place within the iron core of the rotor. Both solutions require a larger conductive cross-section, so that the space requirement is also larger than would normally be required by the cage of the simple short circuited rotor. Since the increased space is required precisely in the magnetically active part of the machine, this involves not only the increase in dimension of the active parts, but also an increase in the general dimensions of the machine.
It has been attempted to increase the losses at start by means of the stray flux of the bars of the rotor or of the short circuit ring. Such a solution has been described e.g. in the German Patent DE-PS No. 870 449. With this solution an iron ring has been arranged beneath the short circuit ring in such a manner that there is contact between the two rings. The aim of the arrangement has been to increase considerably the uneven current distribution arising in the short circuit ring during start and to increase the losses in such a manner. The increase of the uneven current distribution has certain limits, and in addition, it involves a considerable danger, since the associated uneven heating results in a damaging thermal dilatation and may lead to the melting of the solder holding the bar and the ring.
Due to the complicated structural formation or the insufficient resulting loss, these solutions have not seen wide use. The qualitative pattern of the stray flux lines--and even to a lesser extent the quantitative relationships have not been duly found. In addition, the influence of the coil-head or end turn of the stator winding has been absolutely disregarded.
The aim of the invention is to develop a solution, with which the added losses required for increasing the starting torque do not arise within the iron core, but in the field of the end turns, which has a simple construction and can be produced at lower cost.